Method of rectifying halide salt baths



Patented Jan. 24, 1950 METHOD OF RECTIFYING HALIDE SALT BATHS Isaac Laird Newell, Wethersfield, Gonna assignor 1. I

to Heatbath Corporation,

Indian Orchard,

Mass., a corporation of Massachusetts No Drawing. Application October 3, 1947,

Serial No. 777,847

4 Claims. (Cl. 252-71) This invention relates to a method of maintaining the efficiency of salt baths which are composed substantially of halides and are commonly used in the treatment of certain metals. By halides in the present disclosure are meant the elements fluorine, chlorine, bromine, and iodine of group '7 of the periodic table.

Such salt baths as are composed of mixtures of alkali metal and alkaline earth halides are commonly fused and used as media for the heat treatment of steels and other metals. It is well known in the case of steel that after a certain period of use a bath becomes decarburizing to the metal. One object of this invention is to provide a simple, inexpensive and effective method for maintaining the efliciency of the salt baths to eliminate decarburization and to do this without the disadvantages attending prior art procedures'so far as they are known to me.

Said salt baths are also used in the treatment of copper alloys and nickel alloys and when used for a certain period with these metals become alkaline to a degree that will cause pitting, etching and staining of the metal being treated.

In maintaining the efliciency of halide baths it is common in the language of the trade to speak of keeping a bath neutral to the metal. In other Words a neutral bath will neither carburize nor decarburize steel, nor will it cause pitting and etching of other metals which are commonly treated in said baths. The term neutral as used in this disclosure is intended to have this meaning. Further, the process of maintaining this type of bath neutral is called a process of rectification.

When such baths are no longer neutral to the metal being treated, it is customary to discard the baths or to treat the baths with materials such as silica or silicon carbide which would cause the formation of insoluble materials which could be removed as a sludge from the molten salt, but with attendant loss of salt along with the sludge. Such methods also limit production as de-sludging usually requires lowering the temperature below the working temperature. The baths also increase in viscosity due to the materials added.

Another method of treatment heretofore used in the case of steel consists in immersing a carbon rod in the molten bath. This method does not give complete rectification under all conditions and presents some hazards since small pieces of carbon breaking from this rod may come in contact with the metal being treated and cause hard spots, or even fused metal in the high temperature baths.

completely neutralize salt baths and yet are not open to the objections noted above.

I have found that a molten salt bath such as is composed of alkali halides, alkaline earth halides, or a mixture of alkali and alkaline earth halides and which has become alkaline and thus may injure the metal being treated can be rectified and made neutral and nondecarburizing to steel and avoid pitting and etching by treating the molten salt bath with a compound, gaseous atstandard temperature and pressure, substantially inert with respect to the metals being treated, but reactive with their oxides, containing fluorine as a component element and at least one other element capable of forming an oxide neutral or acidic in reaction. By standard temperature and pressure I mean 0 centigradeandf760 millimeters. Y. f

Among the gaseous fluorine compounds found to be suitable as rectifiers of such salt baths are the following: boron trifluoride; silicon tetrafluoride; hydrogen fluoride; carbon tetrafluoride; and other gaseous carbon-fluorine compounds such as dichloro-difluoromethane. Of these I have found boron'trifluoride to be especially suitable for rectification of salt baths. This list is not intended to be complete, but is merely given to show the wide variety of compounds that may be used successfully.

It is believed that the rectification takes place in the following manner:

(1) The compound containing fluorine reacts selectively with the oxides present to reduce them to the elemental metallic state.

(2) The newly formed (nascent) metals react with the fluorine to form new stable, neutral compounds which will be either volatile at the bath temperature or soluble in the molten bath.

(3) The new oxides formed are either neutral or acidic in reaction, and when acidic, they further react with any alkali present to form a new neutral compound.

Herewith are several examples of the rectifying process:

Example I A salt bath composed of 45% potassium chloride and 55% sodium chloride is used for the hardening of a steel containing 0.41% carbon;

My invention makes use of materials which 0.66% manganese; 0.016% phophorus; 0.021% sulfur; 0.24% silicon; 0.98% chromium and 0.18% molybdenum and is rectified by the use of boron trifluoride. The salt bath is fused by heating to the hardening temperature for the steel used, in this case 1550 F. and when chemical analysis ically and microscopically.

Example II A salt bath composed of 77% barlum'phloride and 23% sodium chloride is used to harden steel containing 0.39% carbon; 0.72% manganese; 0.21% phosphorus; 0.38% sulfur; 1.25% nickel and 0 6&% chromium joy-heating at. atemperature of 1525 F. and-,quenching ind-oil. Themoltenesalt iwbathsis kept in.-a neutral condition by rectifying (withcarbonutetrafluoride When chemical analysis l-shows .that :the alkalinity ofthe :lba h is .eXceslsive. .J-Ihewarbon tetrafiuride is bubbled. through the-.molten salt in. a; slowistreamintordere to have sufiieienticontactof thegas with the. molten salt. when-lthe-salteisrkept.-neutralized= by thiseproc ess,

analysis shows the molten salt to contain an excessive amount of alkali, about 3% calculated as barium oxide for this salt and steel. The gaseous silicon tetrafluoride is slowly bubbled through the 5 molten bath until the alkali present is neutral- ..ized. aAfterrheating thertoolsteelfor an hour at :1235'0" F.;.in. therbarium lchl-orider kept-neutral as stated, and quenching the steel in a neutral salt at 1075 F., its surface is found to be substanlO tially the same as the original from the standpoi'nt of-decarburization and surface defects.

These examples are not intended to be comgpletemandathis: method of providing a gaseous -.compound:fon:hal-ide baths used in the treatment ;15.-:offother .than steel=-articles will avoid pitting and etching of such metals as may be treated by said :baths.

What I claim is:

...1.1..'Ihe method of rectifying a molten halide 20 salt bath which consists in the addition to said bath. ofsaucompound, gaseous at standard-temperature andpressure, and composed of-twoeleements, .-(one lfiuorine. .and. the other. chosen. .from the group .consisting of boron, carbon, ,hydrogen,

g5 andcsilicon and regulating-the. addition .of-.said

.compound to .said bath in accordance withthe .de-

gree to which the alkalinity .of .saidbath becomes -.exeessive by .use.

.2. The methoclof rectifying. halide salt baths t t l-w sample ..mentioned: :above when. heated 0 which consists .in. treating .themolten-halide salt ionicnehour at 1525-? Faand quenchedein .oil, will .hetfou-nd to have. a surfaceshowing-:no significant itral condition'by rectifyinglithe molten salt jby itreatingewith'silicon tetrafluoride when chemical .bath l with suflicient boron trifiuoridei ..to. neutralizenthelalkali present. from. excessive use.

.13....The methodrof rectifying halide-salt baths which consists in treating. the molten halide=salt bath. with silicon tetrafiuoride insufricientquan- 40 .tityeto neutralize .the. alkalilpresentwin :said: bath fromexcessive use.

i ISAAC LAIRD. NEW .ELL.

: No'referen'ces cited. 

1. THE METHOD OF RECTIFYING A MOLTEN HALIDE SALT BATH WHICH CONSISTS IN THE ADDITION TO SAID BATH OF A COMPOUND, GASEOUS AT STANDARD TEMPERATURE AND PRESSURE, AND COMPOSED OF TWO ELEMENTS, ONE FLUORINE AND THE OTHER CHOSEN FROM THE GROUP CONSISTING OF BORON, CARBON, HYDROGEN, AND SILICON, AND REGULATING THE ADDITION OF SAID COMPOUND TO SAID BATH IN ACCORDANCE WITH THE DEGREE TO WHICH THE ALKALINITY OF SAID BATH BECOMES EXCESSIVE BY USE. 